High speed computing has become increasingly prevalent in recent years. Networks of computers have become ubiquitous. With the advent of networks of computers there is a growing need for high speed, high bandwidth Ethernet and optical fiber cables between computers and servers. It follows that there is a need for the ability to modulate data at high speed and high bandwidth to apply to the high speed, high bandwidth interconnecting cables. Because of the extremely high bandwidth at optical frequencies, much effort has been placed into development of optical communications using high-coherency laser sources. The effort is directed toward modulating the lightwave output of a laser at high data rates and to impart the modulated lightwave onto an optical fiber cable.
Because of the high bandwidth available when using optical laser sources, for standard binary modulation (on-off keying) of a laser source, the data transmission capacity per laser wavelength is limited by the optical modulation speed. For both directly modulated lasers and externally modulated lasers using optical modulators, the available modulation speed is currently not high enough to meet the requirement for the high-capacity data transmission of 1-10 Tb/s (for many-core or tera-scale computing) as well as low-cost 100 Gb/s Ethernet applications (next generation of the Ethernet).
Multilevel signaling can be used to increase spectral efficiency, i.e. to increase the data transmission rate for a single carrier frequency. There are a number of schemes that can be used, including M-ary pulse amplitude modulation (M-PAM) and multilevel differential phase shift keying (DPSK). While M-PAM is simple to implement optically, it suffers a severe signal-to-noise ratio (SNR) penalty. The SNR is directly related to the bit error rate of the link. For example, for 4-PAM (2×), 8-PAM (3×), and 16-PAM (4× bandwidth increase), the corresponding penalties are 5.5, 10.7, and 15.9 dB as compared to binary on-off keying.
In contrast, the DPSK modulation scheme has a much smaller SNR penalty, while increasing the spectral efficiency. For 8-level DPSK, for instance, the SNR penalty is on the order of 3 dB. In addition, DPSK has greater tolerance to transmission impairments. Despite these advantages, the optical implementation of DPSK is more complicated. Previously, discrete modulators, phase shifters, fiber Mach-Zehnder delayed interferometers (MZDIs), and detectors were used. The discrete component approach not only poses technical challenges, but also is bulky and costly.